With the recent increase in the capacity of the magnetic recording system, attempts have been made to increase recording density of the magnetic recording medium. In order to increase the density of the recording bit on the magnetic recording medium, decrease in the noise of the medium is necessary, and for this, use of smaller magnetization reversal units on the magnetic recording layer is required. Reduction in the size of the magnetic crystal grains constituting the magnetic recording layer has been found effective for such increase in the recording density. However, use of excessively minute magnetic crystal grains is known to invite thermal demagnetization wherein magnetization on the magnetic recording layer becomes thermally unstable. Use of magnetic crystal grains having a uniform size distribution is important to reduce the thermal demagnetization. In other words, size reduction of the magnetic crystal grains simultaneously with the reduction in the grain size dispersion or standard deviation is required in the medium adapted for use in high density recording.
Conventional magnetic recording mediums have been produced by sputtering a seed layer, an underlying layer, a magnetic layer functioning as a recording layer, a protective layer, and the like in this order on a circular glass or aluminum substrate. In the magnetic layer formed by sputtering, size dispersion of the magnetic crystal grains constituting the magnetic layer is large. The size dispersion and the average grain size, however, can be reduced in the case of sputtering by controlling the conditions of the film deposition. Still, the control of the grain size dispersion is difficult, and it is said that the grain size dispersion is limited to the level of about 20%.
An attempt to overcome the need for reducing the size and size dispersion degree of the magnetic crystal grains is disclosed in Patent Document 1, (Japanese Patent Laid-Open No. 2000-48340, corresponding to U.S. Pat. No. 6,162,532) and a document relevant to this Patent Document 1, is Non-Patent Document 1, Science, vol. 287, pages 1989 to 1992 (issue of Mar. 17, 2000).
In Patent Document 1 and Non-Patent Document 1, the magnetic nanoparticles constituting the recording layer are produced not by the conventional sputtering but by a chemical synthesis. In Non-Patent Document 1, FePt alloy (uniaxial anisotropy constant, Ku: 7×106 J/m3) which is a hopeful candidate for the near future high recording density is synthesized in an organic solvent by reacting an iron pentacarbonyl compound (Fe(CO)5) and an acetylacetone platinum compound (Pt(acac)2). According to the Patent Document 1 and the Non-Patent Document 1, magnetic nanoparticles having an arbitrary diameter in the range of at least 3 nm and up to 10 nm with the size dispersion standard deviation of 5 to 10% could be selectively produced by using the chemical synthesis as described above.
The magnetic nanoparticle produced by the chemical sysnthesis as described in the Patent Document 1 and the Non-Patent Document 1 comprises a magnetic metal as indicated 1 in FIG. 1, which comprises either a single magnetic metal element or an alloy containing at least one magnetic metal element. Such magnetic nanoparticle is coated with an organic compound as indicated by 2. This coating of the organic compound improves adhesion both between the magnetic nanoparticles and the substrate surface and between the adjacent magnetic nanoparticles, and there is disclosed that such organic compound coating facilitates the stable production of the ordered array of the magnetic nanoparticles in the formation of the monolayer or multilayer film. FIG. 2 shows a monolayer film of magnetic nanoparticles. In FIG. 2(a), the layer of magnetic nanoparticle layer 5 is formed on the underlying layer or the soft magnetic layer 4 formed on the substrate 3, and the magnetic nanoparticle 1 is covered with the coating 2.
In addition to the role as described above, the coating of the organic compound is believed to play an important role of improving the storage stability of the colloid solution of the magnetic nanoparticles. The presence of the organic compound coating between the magnetic nanoparticles in the resulting film is also believed to reduce the magnetic interaction between the adjacent magnetic nanoparticles. This phenomenon may be similar to the phenomenon found in the medium having the layer of CoCrPt, CoCrTa, or the like formed by sputtering wherein Cr segregated layer is formed at the boundary of the magnetic crystal grains.
Typical organic compounds used for the coating in the Patent Document 1 are organic materials containing a long chain organic compound represented by the formula: R—X wherein R is desirably a member selected from straight and branched hydrocarbon and fluorocarbon chains containing 6 to 22 carbon atoms, and X is desirably a member selected from carboxylic acids, phosphonic acids, phosphinic acids, sulfonic acids, sulfinic acids, and thiols, among which oleic acid being mentioned as the most desirable for use as the coating.
Non-Patent Document 1 describes that, when the recording layer comprising magnetic nanoparticles formed was subjected to a high temperature heat treatment at about 560° C., the coating of the organic compound such as oleic acid did not evaporate, but became carbonized as indicated by 6 in FIG. 2(b) and remained around the magnetic nanoparticles. Such carbonized organic substance remaining between the magnetic nanoparticles is believed to contribute for the reduction of the magnetic interaction between the magnetic particles. Non-Patent Document 1 also describes that crystallographic structure of the FePt magnetic nanoparticles changes by the heat treatment from the fcc structure at the time of its chemical sysnthesis into the ordered structure L10. In the case of FePt, magnetism is not found in the fcc structure, and ferromagnetism is developed when it takes the ordered structure. It is to be noted that the magnetic field was not applied in the heat treatment after the film formation. Accordingly, the easy axis of magnetization of the magnetic nanoparticles is believed to be randomly oriented.
In the technology described in Non-Patent Document 1, the nanoparticle layer formed is subjected to a high temperature treatment at about 500° C. to 600° C. to thereby convert the nanoparticle crystal structure from fcc structure to L10 ordered structure to thereby magnetize the nanoparticles to the degree sufficient for use as a recording medium. As a result of such high temperature heat treatment, the nanoparticle layer experiences disturbance in the array of the nanoparticles as well as agglomeration of the nanoparticles, and when such nanoparticle layer is used in a magnetic recording layer, the layer suffers from an insufficient flatness. The high temperature heat treatment also results in the undesirable deterioration of the underlying layer, the soft magnetic layer, and the like between the nanoparticle layer and the substrate. In spite of the high magnetization degree of the nanoparticle layer after the high temperature heat treatment, it is difficult to use such nanoparticle layer in a magnetic recording medium wherein the substrate is actually rotated for the reading and writing of the information by the read head.
On the other hand, in the technology described in Patent Document 1, the easy axis of magnetization of the magnetic nanoparticles constituting the recording layer is randomly oriented, and orientation of the easy axis of magnetization in a particular direction such as in-plane direction of the medium or thickness direction of the medium is difficult. As a consequence of such difficulty, the resulting magnetic recording layer suffers from inferior magnetic properties compared to the conventional in-plane recording or perpendicular recording medium.